Design and Implementation of a Stand-Alone Photovoltaic Road Lighting System

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1 Design and Implementation of a Stand-Alone Photovoltaic Road Lighting System Jin-Maun Ho Jia-Liang Hsu SM IEEE Department of Electrical Engineering Chung-Yuan Christian University Chung-Li, Taiwan, R.O.C ho@dec.ee.cycu.edu.tw Abstract The solar energy is one of the most promising renewable energy and the using of high brightness Light Emitting Diode (LED) for lighting system has become a trend recently. This paper presents a stand-alone solar electrical power system which can supply power to LED for road lighting. This paper proposes an optimal design of a stand-alone photovoltaic (PV) road lighting system. It has two purposes:the first, to calculate the optimal solar cell and battery capacities under different environmental conditions, such as climate, latitude, and location for effective lighting. The second, to design of a solar power generation system for supplying two sets of LED lighting system, which can avoid the affection of shade and make the road lighting system operated effectively. Finally, in order to verify the road lighting system feasibility, this paper presents a pseudo place like remote mountainous area in Nantou, Taiwan and an actual place in Chung Yuan Christian University to design a stand-alone PV road lighting system each for comparison. The experimental results verify the feasibility of this design. eywords: Solar cell Road lighting LED I. Introduction With the advanges of no noise, no distribution wiring, less maintenance cost, no electricity bills, and etc., the usage of solar electric power has being very popular. Its application for lighting is just one of the examples. For lighting application, the system consists of solar cells, battery, lighting fixture and with a connection to utility in general. The design considerations usually are not including how to reach the best choice of capacities among solar cell, battery and load for a system which has a back up from utility. However, in remote areas, such as high mountains, deserts and islands, a stand-alone solar power system for lighting is very attractive and suitable since it is cost effective. But a stand-alone solar power system for these places need special considerations. The irradiation of sun is changing with seasons, times, locations and weather conditions, therefore, a good design of stand-alone solar system must consider these and then calculate the capacities of battery and solar cell for a presetted load. This paper presents a design method and the procedure to calculate the needed battery capacity for a stand-alone photovoltaic load lighting system. II. Independent solar road lighting system design The conceptual system and its equivalent circuit for this study is shown in figure 1. (a) conceptual PV road lighting (b) equivalent circuit Figure 1 A stand-alone solar lighting system PV can be installed in any places, but the system design depends on the installation site. This section will discuss the consideration factors and design steps for a stand-alone PV load lighting system, and use a design example in a remote area in Taiwan to illustrate the design. 2.1 The standard daily sun radiation and PV generation in Taiwan Table 1 shows the statistic data of sun radiation and generating capacity in some place in Taiwan [1-3]. The estimating PV generating capacity is as in equation 1: E = H P (1) p A AS where, E : estimated generating capacity (WH/day). p H A : solar radiation in interested place. : a factor depends on changes in insolation, panel cleanness, temperature variation, circuit efficiency, battery capacity, and etc. It s values are between 0.6 and 0.85, and 0.7 is used in here. PAS : standard generating capacity (W) per m 2 solar panel, that is 1.0 in Taiwan. ISBN:

2 Place Table 1 Estimated PV generating capacity per m 2 (1 W) panels installed in Taiwan Altitude (Meters) Average annual solar radiation (kj/m 2 day) Average annual solar radiation (kwh/m 2 day)h A Average annual solar generation (kwh/m 2 day) E p , (Banqiuo) (Danshui) , (Anbu) , , (jhuzihu) , eelung , Hualien , Ilan(Suao) , Ilan 7.2 9, inmen , Penghu , Tainan , Tainan (Yongkang) , aohsiung , Chiayi , Taichung , Chiayi (Alishan) , Taitung (Dawu) , Chiayi (Yushan) , Hsinchu , Pingtung (Hengchun) , Taitung (Chengging) , Taitung (Lanyu) , Nantou (Sun Moon , Lake) Taitung , Taichung (Wuqi) Lienchiung (Mazu) , , System design A PV system design includes two aspects, the capacity design and the electrical circuit design, namely. The capacity design is to figure out the needed solar panel arrays, battery arrays and matching loads for year round effective operation of a stand alone system. The electrical circuit design is to design a must suitable electrical circuit to meet the requirement and this will be mentioned in section III. The design flow chart is as in figure2 [4]. Figure 2 capacity calculation flow chart for battery Equation 2 depicts the relation D I L C = (Ah) (2) U where I L = I T and U is chosen as 0.8 for deep discharge type battery and 0.5 for others. However, equation 2 only show a rough approach, for more accuracy, one should consider the influence of operation temperature and discharging rate on the capacity as shown in figure 3. Therefore equation 2 is modified as equation 3 D I L C = (3) U α R where α is temperature modification factor and is less than 1.0 for temperatures lower than 25 C. R is overall circuit modification factor and has been chosen as 0.62 in this system. Capacity (%) Temperature ( C ) Discharging rate Figure 3 Needed battery capacity versus temperature and discharging rate Design of PV array module For satisfying the average daily demand, a PV module design flow chart of PV is shown in figure Battery capacity calculation The battery should able to supply the load even in a lower than average radiation of solar situation for some successive days. ISBN:

3 The required load current, I, and voltage, V Daily use time T ( hours ) Calculate the daily amount of electricity Q Daily sunshine time TS(h) correction coefficient Necessary solar battery current IS Required parallel array number, Np The needed module The minimum array voltage, VO Required series array number, Ns VD : voltage drop on diode and circuitry. V : voltage decreased by temperature increasing of module Check the design of battery capacity with the design of PV module Once the capacity of battery and PV module have been calculated by equation 3, 4, and 7. There is a checking requirement to ensure a good consideration of both designs. Firstly, check the battery discharging depth as indicated in equation 8 for preventing over discharging daily total load (Ah) Depth of discharging = (8) capacity of battery (Ah) If the results from equation 8 are not acceptable, (based on the data sheet of battery) then the capacity of battery should be modified until it meet the requirements. 2.3 Pseudo design examples A remote site had been chosen to fulfil a pseudo design, the climate data is an in figure 5. PV array Figure 4 Design flow chart of PV capacity The average output current of PV, Is, can be calculated from equation 4 I L I s = (4) T S where is a modification factor, can be expressed in equation 5 = (5) Where 1 is temperature corrective factor and can be expressed as in equation 6 o = 1 α ( T 25 ) (6) 1 C C T is operation temperature of module and, α is C temperature of coefficient (0.004~0.005 for single and multi crystal cell, 0.002~0.003 for amorphous cell ). 2 : aging factor (0.9~0.95 for single and multi crystal cell, 0.7~0.8 for amorphous cell ). : off maximum power point correction factor 3 (0.9~0.95). 4 : factor of cells in parallel or series (0.95~1.0). : circuit loss factor (0.95~0.98). 5 : charging and discharging loss factor (0.95~1.0). 6 : coulomb efficiency factor (0.9~0.95). 7 The voltage of module is as equation 7 V = V + V + V (7) V V O O F F D : minimum voltage of module. : floating voltage of battery. Figure 5 The average month sun irradiation and temperature of the remote site (Sun-Moon Lake)[3]. Form figure 5, the lowest temperature is around 13 C on January and the highest temperature is around 27 C on August, therefore set the lowest and highest environmental temperature at 10 C and 35 C, respectively. And from table 1, the shortest sunshine month is on April, having hours, the average is 3.7 hours per day, the set shortest sunshine time is 3.5 hours per day. There are the considerations for example 1. With the same conditions except the sunshine time is increasing to 4.7 hours per day as design example 2. With same conditions as in example 1, but self-supporting of battery is increasing to 4 days as design example 3. The calculated capacities of PV panels and battery and other data are shown in Table 2. ISBN:

4 Table 2 Comparisons of design examples NO Example 1 Example 2 Example 3 Result Sunshine 3.5 h 4.7 h 3.5 h hours Daily load 40Ah 40Ah 40Ah Wattage of 48W 48W 48W LED Voltage 12V 12V 12V Hours of 10 h 10 h 10 h usage Maximum discharge 80% 80% 80% depth of battery Supply times 2 days 2 days 4 days The average 0.04 C 0.04 C 0.02 C discharge rate Battery 180Ah 180Ah 360Ah capacity Total PV output A 13.14A A current Minimum voltage of 15.95V 15.95V 15.95V PV operating The number of PV in parallel The total PV 300W 225W 300W capacity Depth of discharge III. Experimental electrical circuit design 68V and maximum power current (Irate) of 4.4A. Microprocessor (PIC18F452) has programs of main subroutine, MPPT subroutine, charging and discharging control subroutine. Figure 7 shows the main control program. Step-up driving IC (AMC3202) and constant current driving IC (AMC7140) [5] of ADDtek are chosen for charging and discharging control, as in figure 8. AD to read the battery charge current control Stop Charging Does charging reading 80%? Switching to charge mode Calculate the battery residual Read solar panels AD To MPPT Whether charged 100%? (C/100) start Solar voltage is less than the set value? Continuous charging Switching to discharge mode Calculate the battery residual Stop discharge Whether the voltage lower than 11.1V Continuous discharge Return to start Figure 7. The main flow chart of load lighting system control Figure 6. Block diagram of system architecture The circuit design is according to the actual load requirement and the real site environmentals. A suitable solar panel is chosen as well as other hardwares. The system consists of eight circuit blocks, namely: (1) solar cell array, (2) feedback circuit, (3) photo coupling circuit, (4) microprocessor, (5) DC/DC converter, (6) IC driving circuit, (7) battery, and (8) LED module, as shown in figure 6. The solar cell array rated at 300W with a maximum open circuit voltage (Voc) of 86.8V, maximum short circuit current (Isc) of 4.8A, maximum power voltage (Vrate) of Figure 8 discharging control circuit ISBN:

5 Ⅳ. Experimental Results The experiments were carried out on May. Figure 9 (a) shows the output voltage and current values, and figure 9 (b) shows the charging voltage and current curves on the same day. The loads are two LED modules. Each module consists of 16LED cells (8 in series and 2 parallel, each with rating at 2.8W, 4V and 0.7A). Either step-up IC is on and constant current IC is off, or step-up IC is off and constant current IC is on, the LEDs do not light, the voltage and current on these situations are shown in figure10 (a) and (b), respectively. Only with both IC on, the output can drive LEDs on operation. Figure11 (a) and (b) show the LED modules operate satisfactory when only use one PV panel to supply two LED modules no matter there is a resistance or not between them. (b) constant current IC operating and step-up IC not operating Figure 10. voltage and current values of LED module with different IC operations (a) voltage and current curves of solar panel (b) charging voltage and current curves of battery. Figure 9.the voltage and current curves of the day (a) direct connection between two LED modules (a) step-up IC operating and constant current IC not operating (b) inserting a 0.5Ω resistor between two LED modules Figure 11. voltage, current, and power waveforms of LED module. ISBN:

6 Ⅴ. Conclusions A stand-alone photovoltaic road lighting system is often not effective due to poor design. This paper proposes a design method based on capacity theory which incorporates with local historic weather data to design a LED road lighting system. Using a remote mountain area as an example, by employing annual sunshine hours, we can calculate the needed capacity of solar photovoltaic power and then calculate the needed battery capacity to reach a optimal design among size of solar panel, battery capacity and load. By this way, applying solar power to road lighting becomes more feasible. And by simulation, we also find using a single solar module to light two separate road lightings is capable even there exists a voltage difference between two LED loads. References [1] Ming Jin Ho, Wen-Sheng Ou, and Jianfu Chen, Taiwan's solar energy design standard solar radiation and associated test specification of the research, collaborative research report Building Research Institute Ministry of the Interior, ROC, December, [2] Ming Jin, Wen-Sheng, buildings, build model solar optimal design of research, Ministry of the Interior Building Research Institute, the ROC, December, [3] Central Weather Bureau, statistics - weather statistics, [4] Shen-hui, and Zuqin Zhen, solar photovoltaic technology, Wu-Nan Books, Republic of China, February, [5] Yang Ling, solar street light of the development, a master's thesis, National Changhua rmal University, ROC, June, [6] ADDtek, AMC3202, AMC7140 Datasheet, ISBN: